Emission characteristics and formation mechanisms of PM2.5 from co-firing of algal biomass and coal

To address the emissions and formation of fine particulate matter (PM2.5) from co-firing of algal biomass (Chlorella (Ch), Sargassum (Sa), and Enteromarpha (En)) and pulverized coal (Huating coal), the emission characteristics were experimentally investigated using a one-dimensional tubular furnace. The results indicate that algae species, the mixing ratio, and the reaction temperature have complex impacts on PM2.5 emissions. Its emission characteristics present either single- or double-peak distribution. The higher temperature and higher biomass fraction accelerate the formation of PM2.5 during co-firing. Different from the reduction in NOx and SO2 emissions, the PM2.5 emission from algal biomass-based co-firing is higher than that of coal alone on the whole, particularly for Sa and En. For En, the maximum PM2.5 concentration occurred at 900 °C with 91.73 mg/m3 when 20% En was co-fired with 80% coal. Meanwhile, the PM2.5 emission yield reached the peak of 553.64 ( × 10−2) mg/g. For Sa, the highest PM2.5 concentration and PM2.5 emission yield appeared when 20% Sa and 80% coal were co-fired at 1000 °C, were 125.30 mg/m3 and 354.41 ( × 10−2) mg/g, respectively. For Ch, the highest PM2.5 concentration appeared when 20% Ch and 80% coal were co-fired at 1000 °C and reached 1.11 mg/m3, but the largest PM2.5 emission yield was 3.21 ( × 10−2) mg/g when 20% Ch and 80% coal were co-fired at 1100 °C. PM2.5 generated from co-firing are mainly formed by vaporization and homogeneous/heterogeneous condensation, as well as fragmentation of internal/external minerals and char particles. In addition, it also depends upon a series of reactions between the alkali metal salt and other incombustible particles. The morphological characteristics of ash residues from Ch co-fired with coal were irregular and fragmented, and the structure was relatively compact, and those for Sa and En co-fired with coal are similar, both of which are irregular massive structures with white crystal particles attached.